Methods and systems for remote detection of gases

ABSTRACT

Novel systems and methods for remotely detecting at least one constituent of a gas via infrared detection are provided. A system includes at least one extended source of broadband infrared radiation and a spectrally sensitive receiver positioned remotely from the source. The source and the receiver are oriented such that a surface of the source is in the field of view of the receiver. The source includes a heating component thermally coupled to the surface, and the heating component is configured to heat the surface to a temperature above ambient temperature. The receiver is operable to collect spectral infrared absorption data representative of a gas present between the source and the receiver. The invention advantageously overcomes significant difficulties associated with active infrared detection techniques known in the art, and provides an infrared detection technique with a much greater sensitivity than passive infrared detection techniques known in the art.

BACKGROUND OF THE INVENTION

The present invention relates to systems and methods for remotelydetecting at least one constituent of a gas via infrared detection. Asystem provided in accordance with the invention comprises at least onesource of broadband infrared radiation and a spectrally sensitivereceiver positioned remotely from the source. The source preferablyfeatures a surface having dimensions at least as large as the receiver'sfield of view at the distance separating the receiver and the source,and the source and the receiver are oriented such that the surface ofthe source is in the field of view of the receiver. The source includesa heating component thermally coupled to the surface, and the heatingcomponent is configured to heat the surface substantially uniformly to atemperature above ambient temperature. The receiver is operable tocollect spectral infrared absorption data representative of a gaspresent between the source and the receiver. The system of the inventionadvantageously overcomes significant difficulties associated with activeinfrared detection techniques known in the art, and provides an infrareddetection technique with a much greater sensitivity than passiveinfrared detection techniques known in the art.

As a background to the invention, much renewed emphasis has been placedin recent years on measuring trace gas constituents of the atmosphere,and researchers continually attempt to develop more sensitive methods toassess the state of the environment. Most countries now regulate orrestrict atmospheric pollution and many have dedicated agencies (such asthe EPA in the United States) to monitor gas-phase pollution levels.Hence; great emphasis is being given to developing reliable, nearreal-time sensor systems to monitor changes of industrial effluents. Avariety of techniques have been described in the literature for using aFourier Transform Infrared (FTIR) spectrometer for remote detection of agas. Remote sensing with FTIRs roughly falls into two rather broadcategories, namely, systems that are “active” in nature versus thosethat are “passive” in nature.

In an active system, the distinguishing characteristic is that there isan active source of infrared (IR) radiation, i.e., an artificial brightsource of IR light, which is focused and collimated to provide ahigh-energy IR beam. Examples of active sources described in theliterature include, for example, an SiC glow bar or a Nichrome wireelement powered by a low-voltage high current DC source. The radiationis collimated using a telescope, and the beam is projected across thedesired area and focused onto an IR detection system. In an alternativeactive method, called a monostatic setup, the source and detectingspectrometer stand together. The propagated IR beam is focused upon aremote retroreflector in a manner whereby the beam is reflected back tothe receiver. The monostatic case is complicated by the fact that thereturned beam must be precisely received by the receiver port in orderto be focused onto the detector. In either case, active systems conferthe advantage that the infrared detector sees a very large temperaturedifference between the background and the target gas plume (i.e., thereis a hot active IR source). Typical sources may operate near 1350 Kelvin(K), and for measurements made near the earth's surface, ambienttemperatures are near 275±40 K, thus providing a temperature contrastbetween sample and background of about 1000 K.

Although active FTIR spectroscopy offers excellent sensitivity, thelogistics of trying to align the sender and receiver telescopes poses aserious problem. Successful employment of the active mode requirescareful co-alignment of the IR sender and receiver telescopes, typicallyto better than 5 arc-sec. For a typical telescope clear aperture ofabout 25 cm (i.e., about 10 inches) and typical separations of 50 to 500meters, it is extremely difficult to align the optical paths of thesender and receiver telescopes using commonly available mechanicalmeans. Moreover, detection is only accomplished over the path defined bythe set-up, and is not easy to readjust by angular sweeps because thesender and receiver telescope bores must not only be parallel, but mustbe co-axial as well, which can be difficult to achieve and maintain,usually requiring radio communication. Monitoring even a slightlydifferent path requires a new alignment, which is cumbersome, requiresremote communications, and is typically very time consuming, oftenrequiring several hours. The analogy has been made that it is similar to“two rifles trying to shoot each other down the barrel,” e.g. requiringalignment to better than 1 arc-min and approaching 2 to 5 arc-sec.

Maintaining both position and orientation is crucial in an active FTIRremote sensing system because an FTIR detector registers only thetime-dependent modulation of intensity. As a consequence, even smallarbitrary intensity fluctuations while recording the interferogramresult in bogus features in the spectrum obtained from the FT algorithm.The active monostatic configuration combines the sender and receiver asa single bore-sighted unit with a retroreflector used to return the IRbeam to the sender, alleviating many of the field alignmentdifficulties; however, sighting to the retroreflector within its FOV canstill be formidable and the method presents other challenges for goodsignal recovery. Since the IR light does not emanate from a pointsource, the divergence of the beam can be a limiting factor since twicethe nominal distance is covered, and the divergences of the outgoing andreturn beams seriously limit the radiation-gathering ability. At largeseparations the retro aperture(s) must be large to reduce theircontribution to overall beam divergence. Finally, the outgoing andreturn beams also need to be separated; the simplest duplexing method isby a beam splitter, meaning that at best 25% of the light gathered fromthe source is seen by the detector. The divergence and reflective lossestypically mean that less than about 10% of the gathered light is seen bythe detector.

The alternative to the active IR detection method is passive FTIRspectroscopy for standoff detection of chemical signatures. The mainadvantage of the passive technique is that it has only a receiver opticand spectral sensor. There is no active source, which eases manylogistical and operational requirements. The technique uses the ambientthermal radiation of the earth as the source of infrared light. If a gascloud is at a temperature that is colder than the backgroundtemperature, an absorption signature is seen (provided the gas has an IRabsorption spectrum, as most do). Should the gas temperature be greaterthan the background temperature, the chemical signature will be seen inemission mode. The passive configuration is sensitive for near real-timeanalysis because the detector (typically, a liquid N₂-cooledsemiconductor at 77 K) is colder than the surroundings and is thuscapable of detecting the incoming IR radiation. That is to say, sincethe detector is at about 80 K, and the earth is typically near 300 K,there may exist a difference in the radiometric output of the cloud andits background, and the detector is cold enough to register thisdifference. When the “on-plume” signature is seen in emission mode, thecloud is at a temperature hotter than the background temperature, andthe chemical signature peaks are upward going, typically atop a broadspectral background due to the instrumental response. To remove thebackground and improve the sensitivity of the technique, an “off-plume”spectrum is normally recorded, and this is subtracted from the“on-plume” spectrum. This can be done simultaneously at the level of theinterferogram providing more sensitive differencing. Ideally, thedifference spectrum has a simple flat background with only the signaturepeaks visible. Should the background temperature be greater than theplume temperature, the same signatures are seen in absorption mode.Clearly, the temperature difference plays a crucial role in the natureof the signal and its sensitive recovery.

In either absorption or emission, however, the greatest limitation ofthe passive technique is usually the lack of significant thermalcontrast between the plume and its background. Even when plume andbackground are at the same temperature it is theoretically possible tostill measure the plume if its emissivity is significantly differentfrom that of the background, because there is still a difference inradiance; however, in practice, the difference in emissivity of thebackground and foreground are similar in scale, so spectral brightnessdifferences are hard to exploit. For passive FTIR there are somecircumstances in which one can anticipate a large ΔT for passivesensing. For example, a stack emission could be known to be at a veryhigh temperature, such as about 200° C., in relation to an ambientbackground. In many cases, however, this cannot be guaranteed, orstack-release temperatures can be highly variable, or possibly evenintentionally changed. In such cases, it would be very advantageous tobe able to guarantee a minimal thermal contrast that is as large aspossible.

Most FTIR work exploits a narrow portion of the thermal IR spectralregion. For example, passive remote sensing techniques are generallylimited to the long-wave infrared (LWIR) atmospheric window in the 1300to 700 cm⁻¹ (7 to 14 μm) range. The CO₂ band at 667 cm⁻¹ and many waterrotational lines generally obscure the region below 700 cm⁻¹, while thewater bending band centered at 1604 cm⁻¹ obscures much of the regionfrom 1300 to 1900 cm⁻¹, thus leaving only a 700 to 1300 cm⁻¹ window forpassive studies. Spectral information in the mid-IR is also useful,especially for monitoring CO and acid halides, but has very low ambientflux making passive detection of the molecules much less sensitive.Although passive FTIRs can monitor a wide diversity of compounds in thethermal IR (LWIR) atmospheric window, and therefore are a natural choicefor air quality testing, they are not sensitive to acid halides and somelight IR active gases such as NO or CO which are emitted by many exhauststacks.

In view of the above background, it is apparent that there is acontinuing need for further developments in the field of remote gassensing using infrared detection. In particular, there is a need forfurther advancement in the development of techniques that are easy touse and provide a sensitivity that is capable of providing usefulinformation to an operator. The present invention addresses these needs,and further provides related advantages.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides novel systems and methodsfor remotely sensing at least one constituent of a gas via infrareddetection. Inventive systems and methods avoid the difficultiesassociated with aligning two telescopes or separating a retro-reflectedbeam as are encountered in active IR detection systems, and provide muchgreater and more reliable sensitivity than passive IR detection systems.

In one aspect, the invention provides a system that comprises at leastone source of broadband infrared radiation, and a spectrally sensitivereceiver positioned remotely from the source. The source preferablyfeatures a surface having dimensions at least as large as the receiver'sfield of view at the distance separating the receiver and the source,and the source and the receiver are oriented such that the surface ofthe source is in the field of view of the receiver. The sourcepreferably provides non-collimated broadband infrared radiation thatfills the field of view of the receiver. The source includes a heatingcomponent thermally coupled to the surface, and configured to heat thesurface to a temperature above ambient temperature. The receiver isoperable to collect spectral infrared absorption data representative ofa gas present between the source and the receiver.

In one embodiment of the invention, the system also includes means forcomparing the data representative of the gas to known gas infraredabsorption spectra to determine at least one characteristic of the gas.For example, the receiver can be operably coupled to a processingsubsystem in which the known gas infrared absorption information isstored. The processing subsystem is operable to perform the comparisonand display the comparison results to an operator. The receiver or theprocessing subsystem is preferably configured to generate an absorptionspectrum representative of the gas. In one preferred embodiment, thereceiver is a Fourier Transform Infrared spectrometer.

In another embodiment of the invention, the heating component isoperatively coupled to a temperature controller. The temperaturecontroller can be configured to receive input from a manual adjustmentmeans, or can receive input from a remote location, for example throughconnection to a remote processing subsystem. In yet another alternateembodiment, the system includes a temperature sensor operable to senseambient temperature. A system including a temperature sensor can beconfigured to control the heating component such that the heatingcomponent heats the surface to a temperature that is a predeterminednumber of degrees higher than ambient temperature sensed by thetemperature sensor. In alternate embodiments, the temperature sensor,the temperature controller, the receiver, or combinations thereof areoperatively coupled to a processing subsystem.

In yet another embodiment, the system also includes a second source ofinfrared radiation positioned at a second location remote from thereceiver. The second source also includes a surface and a heatingcomponent thermally coupled to the surface, the heating componentconfigured to heat the surface to a temperature above ambienttemperature. The surface preferably has dimensions at least as large asthe receiver's field of view at the second distance. The receiver isoperable to collect spectral infrared absorption data representative ofa second gas present between the second source and the receiver, and isconfigured for alternate positioning such that the surface of the firstsource and the surface of the second source are alternatively in itsfield of view. In one embodiment, the receiver is rotatably mounted upona base; and the first and second sources are positioned such that thereceiver is alternately rotatable from a first position in which thesurface of the first source is in its field of view to a second positionin which the surface of the second source is in its field of view, andfrom the second position to the first position. In still otherembodiments, systems are provided that include one or more additionalsources of infrared radiation at alternate positions, and the receiveris configured for repositioning to a position whereby the surface of thefirst source, the second source or the surface of one or more additionalsources is in the field of view. The receiver is operable to alternatelycollect spectral infrared absorption data representative of a gaspresent between the receiver and the surface of the first source, a gasbetween the receiver and the surface of the second source, and a gasbetween the receiver and the surface of the one or more additionalsources.

In another aspect of the invention, there is provided a system forremote monitoring of a gas that includes a plurality of spectrallysensitive receivers positioned at separate receiver locations. Each ofthe receivers includes an optical component defining a field of view.The system also includes at least one source of broadband infraredradiation, and optionally multiple sources, positioned remotely fromeach of the receivers. Each of the sources comprises a surface and aheating component thermally coupled to the surface; wherein the heatingcomponent is configured to heat the surface to a temperature aboveambient temperature. Each receiver is positionable such that at least aportion of a source surface is in its field of view, and each of thereceivers is operable to collect spectral infrared absorption datarepresentative of a gas present between the respective receiver and thesource. The system also can include a processing subsystem coupled toeach of the receivers. The processing subsystem is operable to comparedata from each receiver to information stored in the subsystem toprovide comparison results to an operator. It is also optionallyoperable to perform multiple additional operations, or combinationsthereof, including, for example, receiving temperature information fromtemperature sensors, calculating temperature differences, controllingheating components of the respective sources to adjust temperaturedifferences, controlling movement of receivers, including movements topoint the optical components to different sources and movements tooptimize alignment, and the like.

The present invention also provides a novel method for remotely sensingat least one constituent of a gas. In one aspect, the invention providesa method including (1) providing a spectrally sensitive receiver at areceiver location, the receiver including an optical component defininga field of view; (2) providing an extended source of broadband infraredradiation at a source location separated from the receiver location, thesource comprising a surface and a heating component thermally coupled tothe surface; wherein the heating component is configured to heat thesurface to a temperature above ambient temperature; and wherein thesource and the receiver are oriented such that at least a portion of thesurface of the source is in the field of view; (3) collecting spectralinfrared absorption data with the receiver, the data beingrepresentative of a gas present between the source and the receiver; and(4) comparing the data representative of the first gas to known gasinfrared absorption information to determine at least one characteristicof the first gas. In one manner of practicing the invention, thereceiver is operably coupled to a processing subsystem operable tocompare the data to information stored in the processing subsystem to anoperator. In another manner of practicing the invention, the collectingcomprises generating an absorption spectrum representative of the gas.

In another form of the invention, the method further includes (1)providing a second blackbody source of infrared radiation at a secondsource location remote from the receiver; (2) moving the receiver to aposition whereby the second source is in the field of view; (3)collecting spectral infrared absorption data with the receiver, the databeing representative of a second gas present between the second sourceand the receiver; and (4) comparing the data representative of thesecond gas to known gas infrared absorption information to determine atleast one characteristic of the second gas. In still another embodiment,the method further includes alternately aligning the receiver with thesurface of the first source such that the surface is in the field ofview and with the surface of the second source such that the secondsource is in the field of view, and repeating the collecting andcomparing at the alternate positions at predetermined intervals. Instill other embodiments, the method further includes providing one ormore additional sources of infrared radiation at alternate positions;alternately aligning the receiver with the surface of the first source,the second source and one or more additional sources; and repeating thecollecting and comparing at the alternate positions at predeterminedintervals.

In an alternate manner of practicing the invention, there is provided amethod for remotely analyzing a gas that includes: (1) obtainingspectral infrared absorption data from a spectrally sensitive receiver,the data being representative of a gas present between the receiver anda remotely positioned extended source of broadband infrared radiation;and (2) comparing the data to known infrared absorption spectra todetermine at least one characteristic of the gas; wherein the sourceincludes a surface thermally coupled to a heating component, the heatingcomponent configured to heat the surface to a temperature above ambienttemperature; wherein the receiver includes an optical component defininga field of view; and wherein the receiver is oriented such that at leasta portion of the surface is in the field of view.

The invention also provides a method for remotely analyzing a gas thatincludes (1) providing a spectrally sensitive receiver at a receiverlocation, the receiver including an optical component defining a fieldof view; (2) providing an extended source of broadband infraredradiation at a source location separated from the receiver location, thesource comprising a surface and a heating component thermally coupled tothe surface; wherein the source and the receiver are oriented such thatat least a portion of the surface is in the field of view; and whereinthe heating component is configured to heat the surface to a temperatureat which the surface emits sufficient infrared radiation in the MWIRregion of from about 1850 to about 2300 cm⁻¹ to produce a detectablesignal in the MWIR region when a gaseous compound that absorbs radiationin the MWIR region is present; (3) collecting spectral infraredabsorption data with the receiver, the data being representative of agas present between the source and the receiver; and (4) comparing thedata representative of the first gas to known gas infrared absorptioninformation to determine at least one characteristic of the first gas.

Inventive systems and methods offer superior sensitivity to passivemethods by providing a guaranteed large thermal contrast, whileminimizing the significant alignment and stability impediments of theactive measurements.

Further forms, embodiments, objects, features, and aspects of thepresent invention shall become apparent from the description containedherein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic plan view diagram of one embodiment of a remotegas detection system according to the present invention.

FIG. 2 is a schematic plan view diagram of another embodiment of aremote gas detection system according to the present invention.

FIG. 3 is a schematic plan view diagram of another embodiment of aremote gas detection system according to the present invention.

FIG. 4 is a schematic plan view diagram of another embodiment of aremote gas detection system according to the present invention,featuring multiple IR sources.

FIG. 5 is a schematic plan view diagram of another embodiment of aremote gas detection system according to the present invention,featuring multiple receivers and multiple IR sources.

FIG. 6 is a schematic plan view diagram of a measurement configurationused to compare passive, active, and inventive measurements as describedin the Examples.

FIG. 7 depicts a typical plume trajectory when the manlift waspositioned 209.5 cm (82.5 in.) downwind from the release stack, asdescribed in the Examples.

FIG. 8 is a schematic diagram of a thermocouple array as described inthe Examples.

FIG. 9 depicts the intensity of the IR signal measured using threedifferent techniques, as described in the Examples. Each of the threespectra is a “background” spectrum, i.e., with no analyte gas flowingfrom the stack. Trace 701 corresponds to the active measurement, trace702 to a purely passive technique, and trace 703 to an inventivetechnique.

FIGS. 10-12 set forth sample and background spectra in the MWIR for eachof the three different techniques: telescope TL19 (FIG. 10), griddleGR19 (FIG. 11), and sky SK19 (FIG. 12). Note the relative changes ofy-axis signal intensity, as seen in FIG. 9. The background spectra havebeen vertically offset for clarity. The CO band is centered at 2143.3cm⁻¹.

FIG. 13 sets forth LWIR spectra 711, 712 recorded using the inventiveand purely passive techniques, respectively. The spectra are GR21/GR19for the inventive techniques and SK21/SK19 for the passive measurements,respectively. The baseline (no analyte) spectra are particularly evidentat one location 711A, 712A in each spectrum. The SF₆ peak can be seen at948 cm⁻¹ in both the inventive and passive spectra.

FIG. 14 depicts relative spectrometer response when viewing the passiveand inventive sources, i.e., the sky and griddle, respectively.Measurement conditions were identical and the y-axis is the same forboth. The inset shows the 1700-2700 cm⁻¹ region expanded for clarity,with H₂O and CO₂ interfering absorptions noted on plot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to preferred embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

The present invention provides novel systems and methods for remotelydetecting at least one constituent of a gas via infrared detection. Asystem provided in accordance with the invention comprises at least oneextended source of broadband infrared radiation and a spectrallysensitive receiver positioned remotely from the source. The receiver isoperable to collect spectral infrared absorption data representative ofone or more gas present between the source and the receiver. As usedherein, the term “extended” is used to indicate that the source has aradiation-emitting surface that has dimensions of sufficient size tofill a significant portion the field of view of the receiver duringoperation of the system, i.e., at a specified distance between sourceand receiver. In one preferred embodiment, the dimensions of the surfaceare of sufficient size to fill at least about 50% of the field of viewof the receiver during operation of the system, more preferably at leastabout 75%, and still more preferably 100% of the field of view. Anextended source, in contrast to a point source, does not requirecollimating optical equipment to broaden its dimensions in order to besuitable for use in connection with the invention. The source is apreferably a broadband source. In addition, the source is preferably ablackbody source. As used herein, the term “blackbody” is intended torefer to a characteristic of a surface whereby a majority of incidentinfrared radiation is absorbed by the surface. The source alsopreferably has a relatively high emissivity, i.e., an emissivityapproaching 1.0. The system of the invention overcomes significantdifficulties associated with active infrared detection techniques knownin the art, and provides an infrared detection technique with a muchgreater sensitivity than passive infrared detection techniques known inthe art.

In an inventive system, such as system 1 set forth in FIG. 1, source 10is positioned at a distance D from receiver 20. Source 10 includessurface 12 and heating component 14 thermally coupled to surface 12.Heating component 14 is configured to heat surface 12 to a temperatureabove ambient temperature. Heating component 14 preferably heats surface12 substantially uniformly. The term “substantially uniformly” is usedherein to refer to a uniformity of heating whereby the temperatures ofdifferent areas of the surface do not vary by more than about 30° C. Thetemperatures of different areas of the surface more preferably do notvary by more than about 20° C., still more preferably by more than about10° C. While not necessary to the practice of the invention, the surfacein a preferred embodiment is a substantially flat surface.

In one embodiment of the invention, distance D between the infraredsource and the receiver is at least about 5 meters. In anotherembodiment, the distance D is from about 20 meters to about 300 meters.In yet another embodiment, the source is positioned from about 40 toabout 250 meters from the receiver. In still another embodiment, thesource is spaced from about 50 to about 200 meters from the receiver. Inone preferred embodiment, the distance is from about 75 to about 125meters. It is, of course, understood that the present invention is alsosuitable for use where alternative distances separate the source and thereceiver, and it is within the purview of a person of ordinary skill inthe art, in view of the present description, to select a suitabledistance for a given application.

Although it is not intended that the invention be limited by the type ofheating component selected for use in accordance with the invention, oneexample of a heating component that finds advantageous use is anelectric heating element as found in many household cooking appliances,such as, for example, a pancake griddle. Indeed, in prototype testing ofthe present invention, described in the Examples, the source selectedfor use in the inventive prototype was a modified commercial pancakegriddle. It is, of course, not intended that the invention be limited toa heating component of this type. Another type of heating component thatis contemplated by the invention is a radiator-type component definingflow paths for passing heated fluid in thermal contact with the surface,thereby heating the surface. In some applications of the invention, theheating component could utilize heated water. In other applications, forexample, where heating to higher temperatures is required, use of an oilas the heating fluid may be desired. It is well within the purview of aperson of ordinary skill in the art to select and use a suitable heatingcomponent in the practice of the invention.

In one embodiment of the invention, the surface is heated to atemperature of from about 10 to about 300° C. higher than ambienttemperature at the location of the detection system. In anotherembodiment the temperature of the surface is from about 20 to about 200°C. higher than ambient temperature. In yet another embodiment, thetemperature of the surface is from about 50 to about 150° C. higher thanambient temperature. In still another embodiment, the temperature of thesurface is from about 75 to about 125° C. higher than ambienttemperature. It is understood that the sensitivity of the absorbancedata obtained from the inventive system, and thus the ability of thesystem to detect compounds at low concentrations, directly correlateswith the temperature differential between the source and a gas betweenthe source and the receiver. It is therefore not intended that theinvention be limited to the above examples, but rather include othertemperatures that are suitable for achieving the advantageous result ofthe invention, as would occur to a person of ordinary skill in the art.

In another embodiment of the invention, depicted schematically in FIG.2, heating component 14 is operatively coupled to a temperaturecontroller 30. Temperature controller 30 can be of the type that ismanually adjustable by a system operator, or of a type that isresponsive to other types of input. Temperature controller 30 shown inFIG. 3 includes ambient temperature sensor 32, and is operative toreceive ambient temperature information from temperature sensor 32 andcontrol heating component 14 such that heating component 14 heatssurface 12 to a substantially uniform temperature that is apredetermined number of degrees higher than ambient temperature sensedby temperature sensor 32. Alternatively, ambient temperature sensor 32can be independent of temperature controller 30 in other embodiments, orcan be absent. Temperature controller 30 will typically include athermostat configured to maintain the surface at a desired temperature,but may alternatively include other mechanisms for achieving temperaturecontrol. Source 10 can also optionally include one or more surfacetemperature sensors 34 for monitoring the temperature of surface 12 atone or more locations if desired.

Receiver 20 is preferably of a type capable of generating an absorptionspectrum, i.e., a spectrum depicting absorbance information for infraredradiation at multiple selected wavelengths. Such a receiver willtypically include an optical component 22, often referred to as a“telescope,” for focusing infrared radiation into a high energy beam,and components for measuring the relative intensity of radiation atselected wavelengths. The receiver is also capable of generating anabsorbance spectrum from the measured radiation, thus depicting anabsorbance value of a particular gas at each of the selectedwavelengths. A particularly preferred receiver for use in accordancewith the invention is a Fourier Transform Infrared spectrometer. A widevariety of alternative receivers having this type of capability, suchas, for example, dispersive spectrometers and radiometers, can also beused, and a person of ordinary skill in the art can readily select asuitable receiver for use in an inventive system.

Heated surface 12 of source 10 preferably has dimensions at least aslarge as the field of view of receiver 20 at the distance D separatingthe source and the receiver. Thus, when distance D is relatively large,the desired surface dimensions are larger than if the distance D issmaller. In operation, the source and the receiver are oriented suchthat the heated surface of the source is in the field of view of thereceiver. When oriented in this manner, infrared radiation emitted bythe source passes to the receiver as non-collimated radiation. Becausethe radiation is non-collimated, the alignment of the source andreceiver is very simple, requiring only the placement of the sourcesurface in the field of view of the receiver, preferably with thesurface at least approximately normal to the receiver's line of sight.

When the source and receiver are oriented as described, spectralinfrared absorption data is collected that is representative of a gaspresent between the source and the receiver. This data can then becompared to known gas infrared absorption information to determinecharacteristics of the gas, such as, for example, to determine whetheran extraneous gas is present in the atmosphere. It is, of course,understood that anomalies in the spectrum can be detected, such as, forexample, where a foreign gas is present in the atmosphere, even if theidentity of the foreign gas is not identifiable, i.e., is a gas forwhich an infrared absorption spectrum has not been previously obtained.It is, of course, understood that the invention is useful even if theidentity of the gas is not immediately determinable, because it canalert the operator to the presence of an unknown gas in the atmosphere.

It is expected that, more commonly, the invention will allow thespecific determination of the identity of a foreign gas by comparison topreviously-obtained spectral data for a plurality of gases. For example,to maximize the utility of the invention, IR spectra are obtained for awide variety of foreign gases envisaged as being potentially present ina location, and these spectra are used as reference spectra. Spectraobtained during operation of the invention can then be compared to thereference spectra for determination of the identity of the foreign gas.In one preferred manner of practicing the invention, the referencespectra information is stored in memory included in a processingsubsystem associated with the receiver, or in memory accessible by sucha processing subsystem.

In one preferred system, as depicted schematically in FIG. 3, receiver20 is operably coupled to processing subsystem 40. Receiver 20 providesdata corresponding to the gas present between source 10 and receiver 20to one or more processors 44 of subsystem 40 so that the data can becompared to reference spectra information stored in memory 46 orelsewhere. The comparison results can be presented to an operatorvisually with a display, aurally with a loudspeaker, and/or using suchother types of output devices as would occur to those skilled in theart. In this manner, information regarding the identity andconcentration of the gas between source 10 and receiver 20 is providedrapidly, so that, if a foreign gas is present, the information can becommunicated to allow the operator to take appropriate steps inresponse.

Processor(s) 44 can be comprised of one or more components of any typesuitable to process the data received from receiver 20, includingdigital circuitry, analog circuitry, or a combination of both.Processor(s) 44 may be comprised, of one or more components configuredas a single unit, or when of a multi-component form, processor(s) 44 mayhave one or more components remotely located relative to the others, orotherwise have its components distributed throughout inventive system 1.Processor(s) 44 can be of a programmable type; a dedicated, hardwiredstate machine; or a combination of these. For a multiple processor form,distributed, pipelined, and/or parallel processing can be utilized asappropriate. In one arrangement, an integrated circuit form of aprogrammable digital signal processor is utilized.

Memory 46 is included in processor(s) 44, and is arranged for readingand writing of data in accordance with one or more routines executed byprocessor(s) 44. Memory 46 can be of a solid-state variety,electromagnetic variety, optical variety, or a combination of theseforms. Furthermore, memory 46 can be volatile, nonvolatile, or a mixtureof these types. Memory 46 can be at least partially integrated withprocessor(s) 44. Removable processor-readable Memory Device (R.M.D.) 48is also included with processor(s) 44. R.M.D. 48 can be a floppy disc,cartridge, or tape form of removable electromagnetic recording media; anoptical CD or DVD disc; an electrically reprogrammable solid-state typeof nonvolatile memory, and/or such different variety as would occur tothose skilled in the art. In still other embodiments, R.M.D. 48 isabsent. Besides memory, processor(s) 44 may include any oscillators,control clocks, interfaces, signal compensators/conditioners, filters,limiters, Analog-to-Digital (A/D) converters, Digital-to-Analog (D/A)converters, communication ports, or other types of components/devices aswould occur to those skilled in the art to implement the presentinvention.

Subsystem 40 also includes one or more operator input devices 50 and oneor more display devices 52. Operator input device(s) 50 can include akeyboard, mouse or other pointing device, a voice recognition inputsubsystem, and/or a different system as would occur to those skilled inthe art. Operator display device(s) 52 can be of a Cathode Ray Tube(CRT) type, Liquid Crystal Display (LCD) type, plasma type, OrganicLight Emitting Diode (OLED) type, or such different type as would occurto those skilled in the art. In one form, at least a standard keyboardand mouse are included in input device(s) 50, and at least onehigh-resolution color graphic display is included in display device(s)52.

Processing subsystem 40 can also optionally be coupled to acommunication subsystem (not shown) that includes a network servercoupled to a computer network, which optionally includes the internet. Acommunication link can be provided in the form of one or more dedicatedcommunication channels for subsystem 40, a Local Area Network (LAN),and/or a Wide Area Network (WAN), such as the internet. In other words,a server can be remotely located relative to subsystem 40 with thecomputer network providing the link. Indeed, in one embodiment, a serveris coupled to a number of remotely located subsystems with correspondingreceivers. In still other embodiments, more than one server can becoupled to a common receiver and subsystem 40 arrangement.

Temperature controller 30 and temperature sensor 32, whether integratedor separate, can optionally be connected to subsystem 40 in a mannerwhereby temperature information, i.e., ambient temperature informationand/or source surface temperature information, is communicated tosubsystem 40. Subsystem 40 is able to display this information to anoperator of the system, and/or use the information to adjust thetemperature of source 10 or in algorithms used to make comparisons ofinfrared absorbance data from the receiver to reference spectra. Thesurface temperature information can be used, for example, in algorithmsfor determining the concentration of a give gaseous compound in theatmosphere. In an embodiment including ambient temperature sensor 32,ambient temperature information can be used by processor(s) 44 todetermine a desired surface temperature. Alternatively, or in addition,subsystem 40 can be configured to directly control temperaturecontroller 30 based upon predetermined parameters to thereby control IRemission characteristics of source 10. As another alternative,temperature controller can be connected to a separate processor orprocessing subsystem (not shown) for independent control of temperaturecontroller 30, or may be controlled in other manners as describedherein.

Another advantage of the present invention is that it provides thecapability of readily creating additional test paths for testing theatmosphere or other gaseous system in multiple locations. In thisregard, in another embodiment of the invention, depicted schematicallyin FIG. 4, a second source 110 is positioned at a second locationseparated from receiver 20 by a second distance D₂. The second sourceincludes a surface 112 and a heating component 114 thermally coupled tosurface 112 and configured to heat the surface to a temperature aboveambient temperature. The surface 112 of the second source 110 preferablyhas dimensions at least as large as the field of view of the receiver 20at the second distance D₂. The receiver 20 is configured forrepositioning to a position whereby the surface of the second source 110is in the field of view. In this manner, the receiver is operable tocollect spectral infrared absorption data representative of a second gaspresent between the second source 110 and the receiver 20. A person ofordinary skill in the art will readily appreciate that the systemdepicted in FIG. 4 can be used to detect gases at multiple locations byalternately positioning the receiver such that the first source 10 is inits field of view, and collecting spectral infrared absorption datarepresentative of a first gas between the receiver 20 and the firstsource 10; then positioning the receiver such that the second source 110is in its field of view, and collecting spectral infrared absorptiondata representative of a second gas between the receiver 20 and thesecond source 110; and repeating as desired. After collecting spectralinfrared absorption data at each location, the data can be compared toknown gas infrared absorption information to determine at least onecharacteristic of the respective gases, as described above, at discreetpoints in time.

It is readily appreciated that additional sources of infrared radiationcan be provided at additional alternate positions to provide inventivesystems that are capable of remotely detecting gases at a wide varietyof positions about the receiver. Thus, in an inventive system withmultiple sources, the receiver can be sequentially pointed to multiplesources, such that each source is in the field of view of the receiverfor a period of time sufficient to collect spectral infrared absorptiondata representative of a gas therebetween. In a particularly preferredsystem, the receiver is rotatably mounted upon a base, such as base 125depicted in FIG. 4, for example, using a swivel component 126, and thesources are positioned such that the receiver can simply be rotatedabout the swivel component to a plurality of positions in whichalternate sources are in the field of view of the receiver. In a systemin which the heated surface of the source is larger than the field ofview of the receiver, the inventive system is remarkably easy to alignand use. The receiver optics are simply pointed toward the source, andthe system is then ready for use as described herein. This allows therapid realignment of the receiver to monitor gases in multiple locationsin quick succession. Although not necessary to the practice of theinvention, in one embodiment, receiver 20 includes an automaticalignment mechanism 23, as shown schematically in FIG. 3, to aid inpositioning receiver 20 in an orientation whereby source is within thefield of view of receiver 20. For example, a skilled artisan willappreciate that a feedback mechanism can be used in which separatemotors are used to incrementally move the field of view in the verticaland horizontal directions respectively while measuring signal strengthat each incremental position. Incremental movement continues until aposition is reached at which the signal strength is maximized, at whichpoint the field of view is determined to be optimally aligned uponsource 10. A person of ordinary skill in the art will readily appreciatethat processing subsystem 40 can be used to control major movements ofreceiver 20 between predetermined positions, such as between positionsat which different sources are placed in receiver's field of view, bycoupling subsystem 40 to a motor to selectively control receivermovement. Subsystem 40 can also be used to control minor movements ofalignment mechanism 23.

Given a sufficiently powerful computing system for comparing absorbancedata to reference spectra, it is seen that an inventive system canrapidly monitor a large area for foreign gases in a very short period oftime, and can continue to periodically monitor the area over a longperiod of time. An exemplary system is depicted in FIG. 5, in which aninventive system is configured to test gases around perimeter 201, whichcan represent a perimeter around any area to be monitored, such as, forexample, a school, a hospital, a military installation, or the like. Inthis system, receivers 220, 320, 420, 520 are positioned at corners ofperimeter 201, and each receiver is configured to pivot about swivelcomponents 226, 326, 426, 526, respectively to monitor alternateperimeter locations. For example, receiver 220 is oriented such that itis movable between a position at which source 210 is in its field ofview to a position at which source 211 is in its field of view.Likewise, receiver 320 can be alternately pointed to source 310 and 311,receiver 420 can be alternately pointed to source 410 and source 411,and receiver 520 can be alternately pointed to source 510 and source511. In this embodiment, processing subsystem 240 is coupled toreceivers 220, 320, 420, 520 to provide the advantages described above.Subsystem 240 can also be coupled to temperature sensors (not shown),temperature controllers (not shown), communications interfaces (notshown) and the like as would occur to a person skilled in the art inview of the present specification. In this manner, large perimeters canbe continuously monitored for the presence of foreign gases. Theinventive system is particularly suited for automated control. Thesystem is readily configured to provide for automatic rotation anddetection steps, which can be controlled in accordance with apre-programmed sequence of steps. It is well within the purview of aperson of ordinary skill in the art to program an inventive system thatis capable of following such a sequence, collecting spectral data,making data comparisons, and recording and/or transmitting informationregarding the gas or gases to an operator located proximal to or remotefrom the receiver.

A person of ordinary skill in the art will appreciate that a remotedetection system as described herein provides significant advantagesover passive remote detection systems described in the prior art bycreating a relatively large temperature difference for the plume versusthe background, thus guaranteeing a much higher probability of foreigngas detection. A prototype system of the present invention has been usedto make a direct comparison of the relative sensitivities of theinventive system to a conventional passive detection system and aconventional active detection system. Using identical acquisition timesand parameters, and using carbon monoxide as the subject analyte, adirect comparison was made between the sensitivity of the techniques.Such a sensitivity comparison cannot cover all possible scenarios, asthe results will depend on the wavelength region, sample of interest,and several measurement conditions, such as the plume and backgroundtemperatures; however, averaged over 5 or 6 measurements, the inventivesystem was found to be approximately 30 times more sensitive thanpassive FTIR where the source used in the inventive method had atemperature of about 80° C. higher than ambient. Specifically, for thesescenarios, described in greater detail in the Examples, the inventivemethod had a minimum detection limit of about 79 ppmV, whereas thepassive method had an average minimum detection limit of 2300 ppmV. Thedirect comparison of the three techniques can be seen in the spectrathemselves. As discussed further below, FIGS. 10-12 present the spectrameasured for active, passive (i.e., sky), and inventive systems. Thebackground spectra 704, 706, 708 were all measured with the CO flowturned off, and the corresponding spectra 705, 707, 709 are the dataobtained with CO gas flowing. In the passive measurement, the gas plumeis hotter than the background, so the peaks are upward-going. For theactive and inventive methods, the CO peaks are seen in absorption mode.

For cases where there is no temperature difference between the plume andambient temperature, the inventive method is infinitely better than thepassive technique because the engineered temperature difference createsthe measurement sensitivity. For passive measurements, in general, toobserve such a species in emission mode at these (5 to 20 μm)wavelengths would require the gas to be extremely hot, perhaps tens ofdegrees Kelvin above ambient temperatures, depending on spectrometersensitivity. For civilian defense purposes, such as the release ofpollutant, toxic industrial material (TIM), or agent gas in an enclosedstructure, where the release species are more likely to be released nearroom temperature, the passive technique would not provide an acceptabledegree of sensitivity. In contrast, the inventive method produces alarge, fixed temperature difference, resulting in tens to hundreds oftimes better sensitivity, especially for gas plumes near ambienttemperatures. Due to their low heat capacity, gas plumes thermalizesurprisingly quickly. Moreover, alignment is simplified in an inventivesystem due to the large infrared signal generated by the source. Thedetecting spectrometer or radiometer simply has to be aimed at thesource, and it is easy to optimize Fourier Transform Spectrometers on areal-time interferogram. The interferogram in the inventive techniquewas found to be 2 to 3 times larger than when the telescope was pointedoff-target, i.e., away from the source.

A person of ordinary skill in the art will readily appreciate that useof the present invention results in significant simplification ofsoftware needs as compared to a passive FTIR system, which must accountfor both emission and absorption features depending on the relativeplume temperature. The passive IR software must thus analyze for eitherof two entirely different types of features. In contrast, because theinventive technique creates a significant temperature difference betweenthe background and the intervening gas, the spectral features are almostalways in absorbance, and standard spectral analysis (using theBeer-Lambert law) is enabled. This not only greatly simplifies thedetection algorithm, but also greatly reduces the processing time neededto compare a test spectrum to reference spectra in an associated datalibrary.

Moreover, the constant large radiative field removes the requirement fora high dynamic range required in a passive system. Specifically, thedetector can be better configured to handle more constant light levelssince the broadband source used in accordance with the invention can bemaintained at a constant temperature, i.e., spectral brightness. Aninventive system with a very hot source (i.e., a source that has atemperature of at least about 20° C. above room temperature) couldperhaps make use of room-temperature detectors such as deuteratedtryglycine sulfate (DTGS).

Another advantage of an inventive system over a passive detection systemis that the heating component in an inventive system can be configuredto heat the surface to a temperature at which the surface emitssufficient infrared radiation in the MWIR region of from about 1850 toabout 2300 cm⁻¹ to produce a detectable signal in the MWIR region when agaseous compound that absorbs radiation in the MWIR region is present.The emission of radiation in this region is a significant advantage ofan inventive system compared to a passive system, wherein very littleradiation in this region is available for detection. As discussed in theExamples below, in the inventive system tested, there was significantlymore IR intensity in this region compared to a passive measurement,which corresponds to a vastly reduced acquisition time for the samesignal/noise ratio. This region of a typical atmospheric IR spectrum isrelatively free of interferants and can provide much useful informationfor environmental monitoring of pollutant compounds. There are severalhighly toxic compounds that can be monitored with far better sensitivityin the MWIR regions since they have few or no absorption bands in thelong-wave IR. In addition, several common small-molecule toxins havetheir strongest signatures in this region. Many of these species can bemonitored only poorly at LWIR wavelengths, and most not at all. Theysimply would not be detected in the LWIR because they have no signaturein that region.

In comparison to an active technique, there is no need in an inventivesystem for sender telescopes or retroreflectors, each of which can coston the order of USD 10,000. In contrast, excellent results were obtainedusing an inventive system in which a modified commercial pancakegriddle, at a cost of about USD 40, was used as the source.Logistically, it is infinitely easier for the detection FTIR system toalign on the griddle source than bore-sighting the two telescopescoaxially. In practice, the two telescopes in an active system can onlybe aligned by skilled operators, typically with two people in radiocommunication, and only with investment of a significant amount of time.For the inventive technique, alignment is especially facile since itremoves the criterion for co-axial positioning, thus requiring only theless stringent angular pointing. Once a coarse alignment has beenaccomplished, steering optics can easily be adjusted so as to maximizethe interferogram in real time. Such a mechanism can also easily employa servo-feedback loop to optimize angular alignment. The inventivetechnique is robust, of minimal maintenance and several paths can bemonitored adding additional (inexpensive) sources thus enablingdetection of inhomogeneously distributed gas plumes. The inventivesystem therefore offers important operational advantages for fence linemonitoring, perimeter scanning or the like. The inventive technique hassuccessfully been demonstrated as a remote-sensing technique that isless cumbersome than active infrared sensing, yet far more sensitivethan passive infrared sensing.

Reference will now be made to specific examples illustrating variouspreferred embodiments of the invention as described herein. It is to beunderstood, however, that the examples are provided to illustratepreferred embodiments and that no limitation to the scope of theinvention is intended thereby.

EXAMPLES Comparison of Inventive Remote Detection Technique to Activeand Passive Detection Systems

A goal of the experimental work described herein was carefulquantification, under well-controlled plume release conditions, as tothe relative sensitivity of the active, passive, and inventive IRdetection techniques. This work was done in an open field release underadverse scintillation conditions (ambient temperature >30° C., heavydaytime turbulence), and therefore represents realistic upper bounds onsensitivity.

Experimental Construct

Measurement Layout

These experiments were carried out using a constructed “short stack,”which was approximately 3 meters (10 feet) tall, for releasing a gasplume, with a mobile manlift platform positioned nearby to supportcertain equipment as described herein. A schematic layout of the topview of the experiment is shown in FIG. 6, and a schematic side view isshown in FIG. 7. Immediately downwind from the short stack 625, a mobilemanlift platform 630 was placed adjacent to the approximate plume path635. The manlift supported a sender telescope, the heated griddle, and athermocouple array.

For detection, receiver 620 included a MIDAC 2401 C FTIR emissionspectrometer coupled to a 35-cm Newtonian telescope that gathered the IRlight and re-collimated it into a 3.8-cm diameter collimated beam. Thelight was reflected into the spectrometer through a ZnSe window andintensity modulated by an interferometer using a Ge/ZnSe beamsplitter.The linear motion interferometer modulated the IR light, and the beamwas focused onto an Infrared Associates 16-0.5 mercury cadmium telluride(MCT) 0.5×0.5 mm detector operating at 77 K. The spectrometer anddetection system were set atop a steel platform for mechanicalstability. The communications link at the site allowed data acquisitionto be synchronized to the releases from the 10-foot short stack. Lightin both the MWIR (3 to 5 μm) and LWIR (7 to 14 μm) atmospheric windowswas of interest; a short-pass filter was used to filter out light ofwavelengths shorter than 2.5 μm, allowing increased electronic gain toavoid some of the “bit noise” associated with the 16-bit 100 kHzanalog-to-digital converter (ADC). The MCT 0.5×0.5 mm detector element,coupled with the spectrometer's field-of-view as further magnified bythe telescope produced a theoretical field-of-view of approximately 1.1mrad. The theoretical image size at 100 meters would thus be 0.11 meters(11 cm), but distortions due to imperfect optics and scintillation aresignificant, and a more realistic image size at this distance isestimated as 0.15 meters.

The spectrometer has an ultimate (unapodized) spectral resolution of 0.5cm⁻¹, and for the passive/active/inventive comparison studies discussedherein for carbon monoxide (CO), the spectral resolution was set at 1.0cm⁻¹ due to the narrow spectral linewidths. The interferograms weretriangular-apodized, zerofilled a factor of four and transformed usingthe Cooley-Tukey algorithm. The software controlling the spectrometerand used for data evaluation was Grams 32 Version 4.11. For bandintegration and relative temperature characterization, the data wereanalyzed using Bruker's OPUS-NT software, Version 3.0. Other parametersare consistent with the FTIR parameter settings outlined by Bertie J E.1998. “Specification of Components, Methods and Parameters in FourierTransform Spectroscopy by Michelson and Related Interferometers,” Pureand Appl. Chem. 70(10):2039-2045.

The release gases used for these studies were selected as SF₆ and CO.The release flow rates for carbon monoxide for the various segments arecompiled in Table 1, infra. SF₆ was used as a “known” to help inalignment due to its especially large absorption cross-section. BecauseCO has only a moderate IR cross-section (as reported in the literature:band strength=1.027×10⁻¹⁷ cm/molecule=276 cm⁻² atm⁻¹ at 273.15 K), andbecause the present spectrometer has only moderate sensitivity, largeconcentrations of CO (2,500 to 10,000 ppmv) were provided for thesesegments. CO was also selected because its resolved ro-vibronic linesallow for an internal calculation of the plume temperature. Anothermotivation in selecting CO is to demonstrate the detectability of aspecies that would be effectively “invisible” in most passive IRsystems, since they predominantly utilize the LWIR window between 7 and14 μm. Carbon monoxide's fundamental vibrational mode lies in the MWIRat about 4.5 μm (2143 cm⁻¹). This extended wavelength capability isfurther discussed below.

All IR data were analyzed using reference spectra from the PNNLNorthwest Infrared database. These data are of high precision andaccuracy both on the wavelength and intensity axes. Each 25° C. spectrumin the database is in fact a composite spectrum derived from a Beer'slaw fit to 10 or more individual measurements, each at a differentburden. The database protocol has removed several artifacts associatedwith FTIR measurements for real-time remote sensing (Johnson T J, R LSams, T A Blake, S W Sharpe, and P M Chu. 2002. “RemovingAperture-induced Artifacts from Fourier Transform Infrared IntensityValues,” Appl. Optics, 41:2831-2839). The database has been furthervetted against the National Institute for Standards and Technology(NIST) quantitative infrared database (Chu P M, F R Guenther, G CRhoderick, and W J Lafferty. 1999. “The NIST Quantitative InfraredDatabase,” J. Res. Natl. Inst. Stand. Technol. 104:59-81). The twodatabases have been found to agree (measured across the entire spectrum)to approximately one percent, which is well within the cumulativeexperimental error of 2.2 percent (Johnson et al. 2002). The focus ofthe present study, however, is to investigate the possibility of using anew inventive method for accurate and sensitive detection.

Extended Broadband Source

Based on the discussion above, a source was desired that could fill thefield-of-view (FOV) of the spectrometer at a distance of approximately100 meters and maintain a substantially uniform temperature across theface of the source at least about 80° C. higher than ambient background.Thus, in a system in which the source and receiver are separated byabout 100 meters, it was determined that the source should have asurface of approximately 15 cm diameter in order to fill the FOV of thepresent spectrometer/telescope, should be capable of maintaining anabsolute temperature requirement of about 110° C., (about 230° F.), andshould have a relatively high emissivity. A commercial “pancake griddle”was identified as being a suitable source upon consideration of theserequirements. Thus, rather than construct a custom source, a commercialpancake griddle was used, represented schematically in FIGS. 6 and 7 asgriddle 610. The griddle is designed for continuous 110° C. operation,and has a face with a high emissivity (low reflectivity), thusapproximating a nearly ideal blackbody source. The griddle was modifiedto act as a source for this experiment in only three ways. First, thegriddle was installed on a vertical mount with a bright frame so as toquickly attach to the manlift. The bright frame 216 allowed it to bereadily sighted at large distances. In addition, the thermostatcircuitry was replaced by a simple “variac” voltage regulator to providea constant voltage to the device. Third, four different thermocoupletemperature sensors were affixed to the griddle in random locations togauge the average griddle temperature as seen by the spectrometer. Thefour sensors' temperature data (along with the thermocouple data) wereread out and stored approximately every 1.5 seconds throughout theafternoon. The griddle temperatures reported here are a simple meancalculated from the four sensors. In fact, each of the four sensorsreported a fairly constant temperature over the several minutes of theIR measurement, but the four temperatures were quite different from eachother. For example, during the 3-minute measurement period correspondingto measurement GR10 (13:48-13:50), the four griddle temperaturesreported were 93.9±3.3, 105.9±2.3, 125.5±2.1, and 125.4±1.5° C. Thedifference in the four temperatures reflects the relative inhomogeneityacross the radiating surface due to the relative proximity of thesensors to the actual heating elements. A typical mean griddletemperature during the course of the afternoon was 110° C. During theafternoon of the testing, the air temperature was about 32.5° C., thegriddle thus creating a temperature difference to ambient of about 80 K.

Besides prototyping the inventive method, an additional objective of theexperiment was to facilitate as direct a comparison as possible betweenthe passive, active, and inventive techniques. Thus, three IR sourceswere situated adjacent to one another on the manlift platform 630: theactive source 640, i.e., sender telescope, the “griddle” broadbandsource 610 used in connection with the inventive technique, and theambient sky in between (i.e., for passive FTIR). The active sender wasan air-cooled SiC glow bar from Bruker mounted at the focal point of a15-cm Newtonian telescope. The griddle has been described. For thepassive FTIR, the focal point was just above the horizon between the twoabove-mentioned sources. During each measurement, the detector telescopewas aimed at the sender telescope, the griddle source, or the sky, insuccession. On the receiver, a simple alignment “rifle” telescope wasused to attain coarse alignment. For fine alignment, the interferogramcenterburst was used to optimize the alignment signal on the active andinventive sources. The FTIR spectrometer was located at a crosswinddistance of 89.2 meters from the stack, approximately 90 meters from themanlift and the sources.

A second experiment was being run in parallel to investigate the rate ofplume thermalization under real-world ambient conditions; these plumethermalization experiments dictated the approximate positions of themanlift. The plume thermalization experiment was designed to measure theplume temperature at a series of different positions downwind from thestack to experimentally determine how quickly the plume cools due tomixing and radiative cooling. In fact; plume cooling was observed to bequite rapid as the manlift was repositioned downstream. The experimentalconstruct, depicted schematically in FIG. 8, consisted of an array 645of sixteen thermocouple sensors 646 formed into a 4×4 grid pattern on 30cm (1 ft) centers. The thermocouples were mounted to the grid frame 647using thin wires 648 so as to sense the plume temperature in situwithout significantly obstructing the plume flow. Using thisthermocouple grid as the plume temperature sensor, the manlift wastherefore positioned at three different downwind positions during thecourse of the afternoon to measure plume cooling: 50.8 cm, 97.8 cm, and209.5 cm (from the metal edge of the stack to the thermocouple array—seearrow 649 in FIG. 7). Note that the plume traversed in front of both thegriddle and sender telescope, and is centered near the third row ofthermocouples. The arrow indicates the length used to measure thedownwind distance of the stack edge to the thermocouple array, which wasvaried by moving the manlift. A total of 39 measurements were made.

The measurements were broken into series, corresponding to the threedifferent positions of the manlift downwind from the stack, withdifferent sources being observed and different gas concentrations beingreleased at each position. At each of the manlift positions, 12 IRmeasurements were made: With the gas flows turned off, backgroundmeasurements were recorded for each of the three techniques, namely 1)passive (detector telescope aimed at open sky), 2) active (detectortelescope aimed at sender telescope), and 3) inventive (telescopepointed at the griddle). The release sequences are compiled in Table 1.

TABLE 1 Experimental Parameters and Acquisition Sequence Release StackPlume Griddle Bkgd Time Mixing Grid-Stack Temp⁺ Temp^(#) Temp Temp FileIntv'l Site Ratio/CO (cm) (° C.) (° C.) (° C.) (° C.) GR10 1348-50Griddle 0: Bkgrnd 50.8 130 68.1 112 ± 13 32.8 SK10 1352-54 Sky 0: Bkgrnd50.8 130 71.1 32.9 TL11 1406-08 Sender 0: Bkgrnd 50.8 133 66.2 32.7 TL121419-21 Sender 10,000 ppm 50.8 133 70.2 33.8 GR12 1423-25 Griddle 10,000ppm 50.8 133 72.7  97 ± 12 33.7 SK12 1426-28 Sky 10,000 ppm 50.8 13373.4 33.4 TL13 1431-31 Sender  5,000 ppm 50.8 133 72.6 33.8 GR13 1434-36Griddle  5,000 ppm 50.8 133 66.9  97 ± 13 33.8 SK13 1436-38 Sky  5,000ppm 50.8 133 73.4 33.3 TL14 1439-41 Sender  2,500 ppm 50.8 133 68.5 33.7GR14 1442-44 Griddle  2,500 ppm 50.8 135 71.3  97 ± 12 33.2 SK14 1444-46Sky  2,500 ppm 50.8 135 67.4 33.0 TL15 1530-32 Sender 0: Bkgrnd 97.8 13362.8 32.9 GR15 1534-36 Griddle 0: Bkgrnd 97.8 133 59.3 109 ± 13 33.5SK15 1538-40 Sky 0: Bkgrnd 97.8 140 61.2 33.1 TL16 1547-49 Sender 10,000ppm 97.8 139 60.0 33.6 GR16 1550-50 Griddle 10,000 ppm 97.8 139 61.0 107± 13 33.5 SK16 1553-55 Sky 10,000 ppm 97.8 140 62.0 33.3 TL17 1556-58Sender  5,000 ppm 97.8 141 58.6 33.1 GR17 1559-01 Griddle  5,000 ppm97.8 142 59.5 109 ± 14 33.6 SK17 1601-03 Sky  5,000 ppm 97.8 142 61.633.8 TL18 1610-12 Sender  2,500 ppm 97.8 138 61.3 33.5 GR18 1613-15Griddle  2,500 ppm 97.8 138 51.1 102 ± 14 33.5 SK18 1636-38 Sky  2,500ppm 97.8 135 61.0 33.3 TL19 1707-09 Sender 0: Bkgrnd 209.5 120 49.1 33.3GR19 1714-16 Griddle 0: Bkgrnd 209.5 137 50.3 106 ± 12 33.5 SK19 1717-19Sky 0: Bkgrnd 209.5 137 49.9 33.4 TL20 1723-25 Sender 10,000 ppm 205.5139 49.9 33.3 GR20 1726-28 Griddle 10,000 ppm 209.5 139 49.5 106 ± 1333.2 SK20 1728-30 Sky 10,000 ppm 209.5 140 50.0 33.3 TL21 1732-34 Sender 5,000 ppm 209.5 134 51.1 33.0 GR21 1735-37 Griddle  5,000 ppm 209.5 14051.4 105 ± 11 33.0 SK21 1737-39 Sky  5,000 ppm 209.5 140 52.0 33.2 TL221741-43 Sender  2,500 ppm 209.5 142 49.9 33.2 GR22 1745-47 Griddle 2,500 ppm 209.5 142 49.9  96 ± 12 33.0 SK22 1478-50 Sky  2,500 ppm209.5 142 51.9 33.1 TL23 1800-02 Sender  5,000 ppm 209.5 *71 40.7 32.4GR23 1802-04 Griddle  5,000 ppm 209.5 *66 40.6 103 ± 13 32.4 SK231804-06 Sky  5,000 ppm 209.5 *63 40.8 32.4 ⁺Stack Temp Sensor 10 in.below rim. ^(#)Temps from thermocouple grid. *Start cart off, onlyblowers on

For the actual chemical measurements, the same sequence was repeatedafter the gases had been activated and flows stabilized. At each manliftposition, nominal CO mixing ratios of 10,000, 5,000, and 2,500 ppmV wereused. At the last manlift position, an additional measurement was madeas the stack cooled to near ambient temperature. To achieve this, theblowers were left on, but the jet start carts (the source of heated air)were turned off. This produced a release temperature near 60° C. at thetop of the stack, as opposed to the 130 to 140° C. stack releasetemperature achieved using the full flow that included the start carts.Table I also reports the stack gas (release) temperatures as recorded bya single thermocouple located about 25 cm (about 10 in.) below theinside rim of the short stack, while the plume was still hot. As thethermocouple data showed, the rate of cooling of this plume was veryrapid, dropping tens of degrees Celsius within one or two meters of therelease.

Experimental Results

Spectral Results

Single-beam reference spectra from each of the three techniques areshown in FIG. 9, where the IR signal intensity has not been calibratedinto radiance (W/m² sr cm⁻¹), but is on the same constant signal scalefor the three different techniques. The three traces 701, 702, 703correspond to the active method (sender), the passive method (sky), andthe inventive method (griddle), respectively. The relative amplitude ofthe signals is notable, since the y-axis is the same for allmeasurements. The apparent amplitude below 700 cm⁻¹ is an artifact dueto the MCT detector nonlinearity; the artifact is especially flagrant atthe high light levels of the active measurement. The measurements weremade in direct succession, e.g. TL19, GR19, and SK19.

As can be seen in FIG. 9, the active technique (trace 701) clearlyprovides the greatest IR amplitude and hence the greatest sensitivity.But in fact, it is the amplitude difference (also called the spectralcontrast) between the background and the sample signals that determinesboth the sensitivity and thus the specificity. We see that species suchas CO (absorbing in the MWIR at wavenumbers greater than 1800 cm⁻¹)would be extremely difficult to measure with passive IR spectroscopy inabsorbance mode due to the lack of IR light at these wavelengths (notethe virtual absence of signal at frequencies greater than 1500 cm⁻¹ intrace 702). In absorption mode, the ambient signal is even less than thereference IR signal, and this creates a scenario of subtracting twosmall numbers from one another, which results in large errors. Toobserve such a species in emission mode at these wavelengths wouldrequire the gas to be rather hot, perhaps tens of degrees Kelvin aboveambient temperatures, or even more depending on the spectrometer's noisefloor, often called the noise-equivalent delta-Temperature (NEAT).

FIGS. 10-12 plot portions of the “sample” and “background” spectra forthe three different techniques from single beam spectra. FIG. 10represents spectra obtained using the active technique; FIG. 11represents spectra obtained using the inventive technique; and FIG. 12represents spectra obtained using the passive technique. Traces 704, 706and 708 are background spectra and traces 705, 707 and 709 are samplespectra. Although the x-domain is the same in all three, the y-axes areon very different scales. In each case, the background spectrum isrecorded under identical conditions, save the absence of the chemical inthe plume. In regards to FIG. 12, we note there are several methodsavailable to extract chemical identity information using passive IRspectroscopy. The direct method simply involves subtracting the“off-plume” from the “on-plume” spectrum. An alternative method that hasbeen proposed involves subtracting a blackbody spectrum at the sametemperature as the plume to increase sensitivity. Yet anotheralternative method “pattern matches” the interferogram (rather than thespectrum) with the corresponding interferogram of the analytes ofinterest. Only that portion of the interferogram is used with thestrongest signatures for the analytes of interest. This method can seekout the most information-laden portion of the interferogram, and alsoavoids the (computationally slow and intensive) Fourier transform, whichcan be very advantageous, especially since the FT can be slow at highspectral resolution.

As opposed to the passive technique, in both active IR and in the systemof the present invention, there is sufficient spectral contrast betweenthe sample and background spectra such that the data can be processed asabsorption spectra A=−log(I/I_(o))=εdC using the background or asynthetic line as the reference spectrum. This further simplifies theanalysis as compared to the passive case where the evaluation softwaremust account for both absorption- and emission-type features, dependingon the relative temperatures. Following Kirchhoff's Law in the limitthat the background and plume have the same emissivities, then noradiance difference will be observed when the plume and backgroundachieve equal temperature. The traces shown in FIGS. 10-12 emphasizethis for the case when ε_(B)≈ε_(PLUME)˜1 (optically thick region), sincethe temperature difference governs the size of the signal.

Verification of Plume Path

The relative IR amplitude of the blackbody radiance spectra seen, forexample, in FIG. 9 indicates that the spectrometer was correctly focusedon the appropriate sources. During the experiment, however, additionalmethods were used to verify to a reasonable degree of confidence thatthe same concentration-time product of the CO plume was in the FOVduring each of the measurement scenarios (about 2.5 minutes each). Foreach of the reported segments, the flow and mass measurementwind-velocity and wind-speed data clearly show that, on average, theplume followed a dispersion pattern similar to that rendered in FIG. 7,which shows a diagram of the manlift and associated hardware when themanlift was positioned 210 cm downwind from the edge of the stack. Theplume is sketched as primarily traversing between the second and thirdthermocouple rows from the bottom, with a greater weight toward thethird row, which is typical of what the meteorological data indicated.More definitively, the plume and thermocouple grid array data also show,by interpolating to the points of highest temperature, that the plumefollowed trajectories similar to that shown in FIG. 7. The spectrathemselves show the presence of carbon monoxide at relatively constantconcentrations, so long as there were no major changes in windvelocity/direction. As such, we may approximate the average CO mixingratio in the plume as relatively constant at some value less than therelease value at the release point at the top of the stack (e.g. 2500ppmV), and further that the plume path as shown in FIG. 7 is areasonable approximation to the average plume path during the dataacquisition periods (about 2.5 minutes each). To be sure, turbulent windpatterns will dilute and disperse the plume more than that expected forsimple Gaussian diffusion during any one measurement. This dilution isfurther discussed below.

The TC data further indicate that the plume cools rapidly, presumablydue to turbulent mixing. This mixing would in turn mean that the COconcentrations have also significantly dropped, even though the plume isnot far from the stack. As expected, during periods of higher windvelocity (5 to 8 m/s), the plume “kneeled over,” traversing the lowersensors in the thermocouple grid and thus being slightly below thegriddle for certain measurements. At lower average wind velocities,typically 2 to 4 m/s, the plume was closer to the top of equipmentmounted on the manlift. The most important point in terms of theinventive technique is that on average a good portion of the plumetraversed the IR sensor's field-of-view. The only exceptions to thiswere for the early-afternoon measurements when the TC grid waspositioned only 50 cm downwind from the stack. In these segments (11through 14) the wind was strong enough and the manlift close enough thatthe plume had its hottest, densest point in either the first or secondrow of the TC grid, primarily the first (lowest) row. As a consequence,little of the plume concentration traversed in front of either thegriddle or the sender telescope, as was further verified by the veryweak CO absorption signal. This was especially true for the GR18sequence, where essentially no CO was detected.

Sensitivity and Linearity Analysis

With reference to FIG. 9, the most straightforward data to evaluate arethose of the active measurements. For these measurements the plumetemperature is clearly much lower than that of the sender telescopesource, so Beer's law can be used to calculate aconcentration-path-length product. The absorbance values are calculatedby using −log(I/I_(o)) where the I_(o) spectrum was obtained accordinglyas TL15 or TL19 with no analyte gas flowing.

The nominal stack release concentrations in these experiments were10,000, 5,000, and 2,500 ppmV, respectively. We observe the linearbehavior between the recorded absorbance and the approximate mixingratio of the release gas. Although the concentrations are very high, thenearly linear behavior and excellent signal/noise ratio are reassuring.In fact, the ratios of the CO band integrals (2230 to 2043 cm⁻¹) for thethree measurements fall in the ratio 2.50:4.65:9.07 (lowest to highest,compare 2.5:5:10). The fact that the ratio declines systematically mayalso indicate multiple scattering effects, e.g. IR re-emission fromoptically dense plumes; such effects can lead to an underestimate whenoptical depths are ≧1. These data have not been smoothed or manipulatedin any fashion, only vertically offset for clarity.

To compare the sensitivity of the three different methods, it is usefulto gauge the magnitude of the absorption signal. In a passive remotesensing experiment, the only value that can be absolutely determined isthe product of (concentration)×(path length)×(temperature difference)[in units of ppm-m-K] as discussed above. However, we know the plumetemperature because of the thermocouple array data; it has also beendetermined from the CO absorption-band profile. It is possible tofurther evaluate the data at hand because the plume temperature in thisexperiment is much lower than that of the sender telescope used in theactive measurement, and we thus calculate using Beer's law to yield aconcentration-pathlength product. The mixing ratio of the plume uponexiting the stack is also known; the only unknown quantities are theexact path length (i.e., the plume diameter) and the dilution factor. Asa first approximation, we take the plume cross-section as a circle. Fromthe thermocouple grid array measurements, it can be estimated as aGaussian distribution within a circle of approximately 0.3 meters (1foot) in diameter. We can thus use the active IR measurements tocalculate a plume mixing ratio, which must clearly be less than thestack-release concentration (either 2,500 or 5,000 or 10,000 ppmV forthese calculations). The results of these concentration estimates arepresented in Table 2, Column 6. An additional important observation isthat at 1 cm⁻¹ resolution, the lines of CO are not fully resolved atatmospheric pressure. This poses a linearity problem since it wassubsequently determined that some of the stronger CO lines wereoptically saturated. Hence, we must treat the CO mixing ratios for the10,000-ppmV segments with some suspicion because of thesaturation/nonlinearity of the stronger lines. For both the active andinventive measurements, the measured integrated band strengths of CO arelisted in Table 2. The integration limits were chosen as 2230 to 2043cm⁻¹ to include the strongest CO lines; spectra were baseline correctedbefore integration.

TABLE 2 Sensitivity Estimates for CO for Active, Passive, and InventiveRemote Sensing. Note that for Segment 18 the strong winds pushed theplume below the FOV for most segments. Est. CO Stack Mixing Stack/Mixing Peak Ratio Calc. Bkgd Ratio HT: CO ∫CO (ppm) Detect File- tempsCO 2179 Band (assume Limit Target (° C.) (ppm) cm⁻¹ (cm⁻¹) 0.3 m) (ppm)97.8 cm TL16- 139/33.6 10,000 0.2164 6.2781 1,950 3.66 Sender GR16-139/33.5 10,000 0.1941 5.8439 1,815 95 Griddle SK16-Sky 140/33.3 10,0001856 TL17- 141/33.1 5,000 0.2012 6.0129 1,867 2.05 Sender GR17- 142/33.65,000 0.1589 4.7400 1,472 118 Griddle SK17-Sky 142/33.8 5,000 1054 TL18-138/33.5 2,500 0.1038 3.0230 939 4.29 Sender GR18- 138/33.5 2,500 0.02020.6462 201 xxx Griddle SK18-Sky 135/33.3 2,500 676 209.5 cm TL20-139/33.3 10,000 0.2211 6.4961 2,018 2.89 Sender GR20- 139/33.2 10,0000.1883 4.2280 1,313 86 Griddle SK20-Sky 140/33.3 10,000 5092 TL21-134/33.0 5,000 0.1256 3.3334 1,036 2.97 Sender GR21- 140/33.0 5,0000.0987 1.8631 579 111 Griddle SK21-Sky 140/33.2 5,000 4420 TL22-142/33.2 2,500 0.0762 1.7913 556 4.85 Sender GR22- 142/33.0 2,500 0.09721.5883 493 88 Griddle Sk22-Sky 142/33.1 2,500 804 209.5 cm, cooler*TL23- 71/32.4 5,000 0.1710 4.3789 1,360 7.97 Sender GR23- 66/32.4 5,0000.1728 3.5873 1,114 160 Griddle SK23-Sky 63/32.4 5,000 >5000 *Start cartturned off; only manifold blowers on for TL23, GR23, and SK23.

In Table 2 we have calculated the concentrations (mixing ratios) for theactive and semi-active measurements using the Beer-Lambert law and anassumed path length of 30 cm. The derived band integrals andcorresponding mixing ratios are tabled adjacent to the signal/noiseratios. The 0.3-m path length is clearly somewhat arbitrary, but wasselected because the thermocouple data suggested. a hottest plumecross-section that had dropped markedly in temperature at any adjacentthermocouple sensor (the sensors were placed at 0.3 meter [1-foot]centers). Moreover, a separate video using an infrared camera alsoshowed that for flow near the manlift, the visible part of the plume wastypically about 0.3 meters (about 1 foot) across. Clearly this is anestimate, and if, for example, we estimate the path length as 0.6meters, the mixing ratios listed in the sixth column would all behalved. In any case, the calculated mixing ratio has been derived byintegrating the 2230 to 2043 cm⁻¹ region, assuming a 0.3-meter path andthen dividing the results by the same integral obtained from the NWIRdatabase reference data. For the inventive measurements the signal andnoise levels were calculated in exactly the same manner as for theactive measurements. To be sure, the temperature difference is not aslarge as with the active method, but the griddle creates a spectralcontrast that allows one to use a Beer's law calculation. The analysisis simplified since it is no longer necessary to also consider theemission scenario as with passive FTIR. For the passive technique,however, the data clearly cannot be processed in the same manner. Thesedata were treated in a manner similar to that used in actual passive IRdata processing, namely the off-plume spectrum was subtracted from theon-plume spectrum, and the signature signal was sought in the differencespectrum.

The active and inventive absorption data were analyzed for signalstrength by calculating the peak height of the R (9) CO absorption at2179.8 cm⁻¹ (Maki G. and J S Wells. 1991. Wavenumber Calibration Tablesfrom Heterodyne Frequency Measurements, NIST Special Publication 821,National Institute of Standards and Technology, U.S. Government PrintingOffice, Washington D.C.). The noise levels were calculated using the2450 to 2400 cm⁻¹ region, which was selected so as to calculate thenoise level in an adjacent spectral domain, yet avoid the effects offluctuations in the CO, CO₂, or H₂O concentrations. For the detectionlimit, the absorption peak height was divided by twice theroot-mean-square (RMS) noise value from the 2450 to 2400 cm⁻¹ region.Although this may be a somewhat generous estimate of the actualdetection limit, more rigorous analytical methods could probably elicitmore from the same data set, for example, using partial-least-squares(PLS) or classical-least-squares (CLS) techniques as described in theliterature. The signal/noise analysis yields a reasonableself-consistent estimate of the sensitivity.

As mentioned above, the passive data were treated in a typical mannerwith the off-plume spectrum subtracted from the on-plume spectrum. In afashion analogous to the active methods, the peak height of the CO2179.8 cm⁻¹ line was divided by twice the RMS noise as calculated in the2400 to 2450 cm⁻¹ region. The results are also presented in Table 2. Toderive an actual detection limit, however, absolute signal strength hadto be assigned to the passive signals. This was done differently thanfor the other two techniques; namely, the stack release concentrationwas used as the “ground truth” value mixing value from the flowmeasurements, rather than the derived concentration.

The sixth column in Table 2 shows the calculated mixing ratios of carbonmonoxide using an estimated path of 0.3 meters. The agreement betweenthe active (TLxx) and inventive (GRxx) measurements is extremely good,showing that the plume path traversed both the griddle source and activesource as sketched in FIG. 7, and also that the methods are consistent,i.e., the inventive technique yields approximately the sameconcentrations as those calculated by the more traditional activemeasurement. The agreement is good with the griddle values 10 to 20%less than the active values. The only set of data for which this is nottrue is for the GR18/TL18 pair. Upon reviewing the meteorological data,however, this set had the strongest average winds (7.2 m/s) for thesegment where the manlift was located 98 cm from the stack. In thiscase, the very strong winds seem to have simply directed the plumepartially under the thermocouple grid (and far below the griddlealtogether). This is further corroborated by all thermocoupletemperatures in GR18 being approximately 5° C. cooler (and barely aboveambient) than in the previous GR17 measurement. The same was found to betrue for the earliest sequences at 50.8-cm downwind, where the passiveand active measurements clearly saw the plume, but the griddle waslocated too high and too far to the left to detect much CO for theprevailing wind conditions. These data are not included in the analysis.

For the active measurements, if we disregard the xx 18 data and averagethe five other analogous sequences we find that the mean detection limitof the five TLxx values is 2.5 ppm, whereas for the GRxx values, theaverage is 79.2 ppm. As expected, the active technique is more sensitivesince there are far more IR photons in the active experiment (so long asalignment is maintained). The active technique thus appears to be about31 times more sensitive for this absorption feature for thisconfiguration. The sender has a 1350 K source whose blackbody outputmaximizes near 1600 cm⁻¹, not far from the 2143 cm⁻¹ absorption of CO.The results are readily interpreted in that IR experiments are typicallydominated by detector noise due to the high background radiation presentat ambient temperatures. Although there are many experimental variants,this provides at least a first gauge as to the relative sensitivity ofthe techniques.

Also important is the relative performance of the inventive techniqueversus passive spectroscopy, also tabulated in the 7th column of Table2. For the passive measurements the detection limits had to becalculated using the stack release ratios of CO. However, for the sameset of measurements, GR16-GR22, the passive measurement yielded anaverage detection limit of 2317 ppmV. Recalling that the inventivetechnique had a detection limit of 79.2 ppmV, this corresponds to asensitivity of the inventive method that is 29 times better than thepassive method for identical measurement conditions. Again, there ismuch variation in how the data are evaluated and interpreted, but thisfactor of approximately 30 times greater sensitivity appears to befairly accurate. The additional greater sensitivity of the activetechnique over the inventive method also appears to be fairly accurate,since this in turn corresponds to about 900 times greater sensitivity ofthe active versus the passive method, which is in accordance with anearlier report by Hergot (Herget W F. 1982. “Remote and Cross-stackMeasurement of Stack Gas Concentrations using a Mobile FT-IR System,”Appl. Optics, 21:635-642), and which is also consistent with anexperimentalist's “rule of thumb” that the active method is typicallyabout 3 orders of magnitude more sensitive than a passive measurement.Interestingly, the sensitivity of the inventive technique operated near380 K appears to be near the geometric mean between the two othermethods.

Table 2 also shows good linear trends and general agreement for theconcentrations as derived by the active and inventive techniques. Thisis true for a series of measurements using any one technique, but alsothat the three techniques quantify the release concentration changeswell. The only exception to this was for the final experiments when thestart carts had been turned off and only the blowers were used to forcethe air carrier gas through the stack. In this experiment, the nominalCO release concentration was 5,000 ppmV. The active and inventivemeasurements agreed quite well and there were measured concentrations of1,360 ppmV and 1,114 ppmV, respectively, both assuming a 30-cm plumewidth. The estimated detection limits were 7.97 and 160 ppmV,respectively, for the active and inventive techniques. For the passivemeasurements, however, the stack release temperature was 63° C., and theambient background temperature was 32.4° C. By the time that the plumetraversed the FOV of the spectrometer, the plume had cooled sufficientlysuch that it could barely be distinguished above the noise in FIGS.10-12. As the thermal contrast between the plume and backgrounddiminishes, the spectral signature approaches zero. Referring back toFIG. 12, the weak CO thermal emission can barely be recognized above theambient background, again emphasizing the advantage of the engineeredspectral contrast of the inventive technique.

It is also of interest to consider the data that were recorded in theLWIR as well as the MWIR region. Although no LWIR-signature chemicalshad quantified releases during these segments, there was an unquantifiedrelease during these segments of a chemical with strong LWIRabsorptions, namely sulfur hexafluoride, SF₆. The SF₆ releases wereprimarily to assist in sighting the plume. Nevertheless, we can easilyobserve the raw signals for both the inventive and passive techniques inthe LWIR region as they are plotted in FIG. 13 and qualitatively gaugethe much stronger signal seen by the inventive method. In the figure,the SF₆ signal is seen to be in emission mode in passive FTIR (trace710) and in absorption mode in inventive FTIR (trace 711). The griddlehas in this case created a temperature background significantly hotterthan the SF₆-laden plume. Indeed, visual inspection shows that the SF₆is at some temperature (likely about 50° C.) that is hotter than thepassive sky background yet significantly colder than the 110° C. griddletemperature. We reiterate that were the background to be at exactly thesame temperature as the plume (assuming approximately equalemissivities), the IR signature of the chemical would not be visible bythe passive technique.

It is worth noting that IR measurements in the LWIR region (7 to 13 μm)are typically made at spectral resolutions lower than 1 cm⁻¹ used inthis experiment, e.g., 4 or 8 cm⁻¹. The resolution one chooses to usedepends on several parameters, including observation distance,measurement time, spectrometer design, or the analyte(s) of interest.The present noise studies are not comparable to all 22 possiblemeasurement scenarios, but the sensitivities one obtains at 1 cm⁻¹resolution and about 150 seconds acquisition time are comparable to thesensitivities one might obtain from a 4 cm⁻¹ resolution measurement andperhaps 5 to 10 second averaging time (albeit at higher specificity).FIG. 10 is a good example from the LWIR region as to how the temperaturedifference determines the magnitude of the chemical plume signal.

DISCUSSION OF EXPERIMENTAL RESULTS

In view of the above, it is seen that the inventive system not only hasa significant sensitivity advantage over conventional passive detectiontechniques, but also has a large spectral advantage compared to passiveFTIR since it further extends the broadband coverage from 650 to 1300cm⁻¹ to an extended range of 650 to about 2800 cm⁻¹, by providing moreIR amplitude in the MIWR region as shown in FIG. 14. This figurerepresents the FTIR spectrometer responses when looking at the differentbackgrounds, namely the sky (i.e., passive technique) and the griddlesource. In FIG. 14, the spectrum produced by the inventive system isdepicted as trace 712, and the spectrum produced by the passivetechnique is depicted as trace 713. FIG. 14 also shows that in the LWIRthere is approximately a factor of 3 greater signal in the LWIR between700 and 1300 cm⁻¹ (near 10 μm), which greatly decreases the measurementtime to obtain the same sensitivity (9-fold in the shot-noise limit).There are a host of compounds that can be monitored in this LWIR region,including many organics such as halogenated organics, polyaromatichydrocarbons, and organophosphorous (pesticide) compounds. The inventivetechnique thus represents an order of magnitude improvement in signalacquisition time in the LWIR region. The faster response of inventivemethods clearly provides a large technological advantage over passivetechniques.

The significance of the increase in signal in the MWIR region near 5 μm(=2000 cm⁻¹) is clearly shown by the spectra in FIG. 14, as enlarged inthe inset, which shows that there is approximately 7 times more IRintensity in this region, which corresponds to a vastly reducedacquisition time for the same signal/noise ratio. The regions between1350 to 1850 cm⁻¹ and 2300 to 2400 cm⁻¹ are normally, not used becauseany signature signals are obscured by H₂O and CO₂, respectively.However, the regions in between, namely the MWIR between 1850 to 2300cm⁻¹ and 2400 to 2800 cm⁻¹ are relatively free of interferants and canclearly provide much useful information for environmental monitoring ofpollutant compounds. Specifically, there are several highly toxiccompounds that can be monitored with far better sensitivity in the MWIRregions since they have few or no absorption bands in the long-wave IR.This list includes many gas-phase corrosive acids, such as HI, HCl, HBr,and SO₂ (which forms H₂SO₄ in the lungs). In addition, several commonsmall-molecule toxins have their strongest signatures in this region,including CO, NO, PH₃, AsH₃, and Ni(CO)₄, as well as cyanide and severalderivative compounds, including HCN itself, cyanogen (NCCN), andtetracyanoethylene. Many of these species can be monitored only poorlyat LWIR wavelengths, and most not at all; i.e., they simply would not bedetected in the LWIR because they have no signature in that region. Asopposed to the typically more sensitive laser methods, the Fouriertransform nature of the inventive method means a wide variety ofcompounds can be monitored simultaneously using both the enhancedsensitivity in the LWIR as well as the “added” spectral domain of theMWIR.

In view of the above description, a person of ordinary skill in the artwill appreciate that the invention provides the advantage of enhancedsensitivity of IR detection of toxic gases compared to passive IRdetection techniques, while also enabling the detection of gases whoseabsorptions lie in wavelength domains not normally accessible to passiveIR spectroscopy. The system thus not only has a broadband sensitivityadvantage, but also removes some of the limitations of passive FTIR byfurther extending the broadband coverage to about 2700 cm⁻¹ rather thanonly 650 to 1300 cm⁻¹. Furthermore, the invention provides theadditional advantage of eliminating the requirement of precise andcontinuous alignment of optical components as is necessary in active IRdetection systems.

The inventive technique finds advantageous use in a wide variety ofapplications, including, for example, signature analysis, high-valuefacility protections and homeland defense applications. For example, apossible location for an inventive system is in an air shaft of abuilding, wherein the system is capable of testing the building's airsupply, while being contained in an all-metal housing. Inventivetechniques advantageously generate and maintain a significanttemperature difference ΔT relative to a gas plume. By providing abackground of sufficiently hot temperature, inventive systems ensurethat the plume temperature is colder than the background temperature forany gas that is released near ambient temperature or has equilibrated tonear ambient temperature (typically about 25° C.). The backgroundtemperature provided by the griddle was typically 110° C., thusproviding a ΔT of approximately 80° C. This corresponds to one to twoorders of magnitude greater sensitivity as compared to typical ambient(passive) measurements. Although not as sensitive as an activetechnique, the pointing/stability requirements of either trying to aligntwo telescopes or separating a retro-reflected beam are removed. Thedetecting spectrometer is simply aimed at the hot source, and alignmentis simplified due to the large IR signal provided by the source (thegriddle is in fact a good approximation to a blackbody at 400 K).

While the invention has been described in detail in the foregoingdescription, the same is to be considered as illustrative and notrestrictive in character, it being understood that only selectedembodiments have been described and that all changes, equivalents, andmodifications that come within the spirit of the invention describedherein or defined by the following claims are desired to be protected.Any experiments, experimental examples, or experimental results providedherein are intended to be illustrative of the present invention andshould not be considered limiting or restrictive with regard to theinvention scope. Further, any theory, mechanism of operation, or findingstated herein is meant to further enhance understanding of the presentinvention and is not intended to limit the present invention in any wayto such theory, mechanism or finding. All publications, patents, andpatent applications cited in this specification are herein incorporatedby reference as if each individual publication, patent, or patentapplication were specifically and individually indicated to beincorporated by reference and set forth in its entirety herein.

1. A system for remotely detecting at least one constituent of a gas,comprising: a spectrally sensitive receiver positioned at a receiverlocation, said receiver including an optical component defining a fieldof view; an alignment mechanism coupled to the receiver and operable tochange the field of view of the receiver by changing the direction inwhich the receiver is pointed; wherein the receiver is operable, via thealignment mechanism, to be sequentially pointed at each of a pluralityof extended sources of broadband infrared radiation such that thesources are within the field of view of the receiver for discreteperiods of time sufficient to collect Fourier Transform spectralinfrared absorption data representative of gas present between thesource at which the receiver is pointed and the receiver, the sourcesbeing separated from one another and separated from the receiverlocation; and further comprising means for comparing the FourierTransform infrared absorption data representative of the gas to knowngas infrared absorption information to determine the identity of thegas.
 2. The system in accordance with claim 1 wherein surfaces of thesources have dimensions of sufficient size to fill at least about 50% ofthe field of view.
 3. The system in accordance with claim 1 wherein asurface of at least one of the sources has dimensions at least as largeas the field of view.
 4. The system in accordance with claim 1, furthercomprising means for determining the concentration of the gas presentbetween the source at which the receiver is pointed and the receiver. 5.The system in accordance with claim 1 wherein the receiver is operablycoupled to a processing subsystem operable to compare the FourierTransform infrared absorption data to information stored in thesubsystem to provide comparison results to an operator.
 6. The system inaccordance with claim 1 wherein surfaces of the sources comprise athermally conductive material.
 7. The system in accordance with claim 1wherein the receiver is configured to generate an absorption spectrumrepresentative of the gas.
 8. The system in accordance with claim 7wherein the absorption spectrum is encoded as an interferogram.
 9. Thesystem in accordance with claim 1 wherein the spectrally sensitivereceiver is selected from the group consisting of a spectrometer and aradiometer.
 10. The system in accordance with claim 1 wherein thespectrally sensitive receiver is a Fourier Transform Infraredspectrometer
 11. The system in accordance with claim 1 wherein thesources are at least about 5 meters away from the receiver.
 12. Thesystem in accordance with claim 1 wherein the sources are from about 20meters to about 300 meters away from the receiver.
 13. The system inaccordance with claim 1 wherein the sources are from about 50 meters toabout 150 meters away from the receiver. 14.-76. (canceled)
 77. Thesystem of claim 1, wherein each of the sources comprises a surface and aheating component thermally coupled to the surface and configured toheat the surface to a temperature above ambient temperature.
 78. Amethod for remotely detecting at least one constituent of a gas, themethod comprising: pointing a spectrally sensitive receiver at a firstsource of broadband infrared radiation for a first period of time;collecting Fourier Transform spectral infrared absorption datarepresentative of a first gas present between the first source and thereceiver; realigning the receiver to point at a second source ofbroadband infrared radiation for a second period of time, the first andsecond sources being separated from one another and from the receiver;collecting Fourier Transform spectral infrared absorption datarepresentative of a second gas present between the second source and thereceiver; and comparing the Fourier Transform infrared absorption datarepresentative of the first and second gasses to known gas infraredabsorption information to determine the identities of the first andsecond gasses.
 79. The method of claim 78, further comprisingdetermining the concentrations of the first and second gasses.
 80. Themethod of claim 78, further comprising comparing the Fourier Transforminfrared absorption data to information stored in a subsystem to providecomparison results to an operator.
 81. The method of claim 78, furthercomprising generating an absorption spectrum representative of the firstand second gasses.
 82. The method of claim 78, further comprisingpositioning the receiver at least 5 meters away from the first andsecond sources.
 83. The method of claim 78, further comprising raisingthe temperature of surfaces of the first and second sources to atemperature above ambient temperature.